In communications networks in which the central Signaling System No. 7 standardized by the International Telecommunication Union ITU or Common Channel Signaling System 7 CCS7, also called SS7, conforming to the ITU-T Q.700 Recommendations and following is used for transmitting messages, the communication between two signaling points SP, i.e. nodes and/or end points of the communications network, takes place via connections which in this context are called signaling links or SS7 links. A node is also referred to as a signaling transfer point STP, whereas an end point, in contrast, is also referred to as a signaling end point SEP.
As described in ITU-T Q.700ff, it is possible in SS7 communications networks to combine signaling links into so-called signaling link sets. Each signaling link set comprises at least one signaling link.
The architecture of SS7 is also described in ITU-T Q.700ff. A distinction is made between different protocol layers, called levels in this context. Levels 1-3 form the Message Transfer Part MTP. A number of Level 4 protocols are defined in SS7 and here are called User Parts UP. Examples of these are the Telephone User Part TUP, the ISDN User Part ISUP or the Signaling Connection Control Part SCCP. Messages of these User Parts are transferred securely by the MTP. In this case a so-called service indicator SI is used in order to distinguish the individual User Parts. The SI is a 4-bit value; the following assignment to the User Parts applies (extract shown):
SIMTP User0Signaling NetworkManagement Messages (SNM)1Maintenance RegularMessages (MTN)2Maintenance SpecialMessages (MTNS)3Signaling ConnectionControl Part (SCCP)4Telephone User Part (TUP)5ISDN User Part (ISUP)6Data User Part (call andcircuit-related messages)7Data User Part (facilityregistration/cancellation messages)8MTP Testing User Part
The MTP, particularly MTP Level 3, also handles the administration of the links. If there are multiple link sets and/or multiple links between two signaling points, load distribution or load sharing, among other functions, is effected by MTP Level 3 on the basis of the so-called signaling link selection field SLS, described in ITU-T Q.704. The SLS value is part of the so-called routing label and has a length of 4 bits.
In known SS7 implementations, load sharing has been carried out until now solely on the basis of the value included in the SLS field. This is insufficient in certain application scenarios, however. In ITU-T Q.700 Section 3.2.1 and in ITU-T Q.704 Section 2.3.1 it is mentioned in each case that the routing of messages, though not load sharing, can also be performed on the basis of the service indicator SI.